Absence of excited triplets in the EPR of the high T_c superconductors and the antiferromagnetic insulator CuO

We have been unable to observe electron paramagnetic resonance signals at temperatures of up to ∼ 570K from either the S= 1 2 Cu2+ ions or the S = 1 excited states of the Cu2+ - Cu2+ pairs in the new Bi2Sr2CaCu2Ox (2212) high Tc ∼80K superconductors, in the YBa2Cu3Ox (123) high Tc∼90K superconductors, or in the antiferromagnetic insulator CuO with TN∼ 230K. No present theory of the finite spin susceptibility in the paramagnetic state is compatible with the absence of EPR. Short-range resonating-valence-bond (RVB) theory gives zero spin susceptibility at low temperatures, but the absence of the S = 1 excited states of pairs appears to eliminate that theory as well. © 1988.